A portable water tank is a temporary collapsible tank designed for the reserve storage of water in firefighting, emergency relief, and military applications. These tanks can be either supported or unsupported. The supported tanks have a steel or aluminum frame and range in size from 600 to 5,000 US gallons (500 to 4,160 imp gal; 2,300 to 18,900 L) or larger by custom design. Portable water tanks are also unsupported such as self-supporting tanks (onion tanks), blivets and pillow or bladder tanks and are available in sizes ranging from 100 US gallons (83 imp gal; 380 L) up to 80,000 US gallons (67,000 imp gal; 300,000 L).